Microbuckle tunnelling in fibre composites

The propagation of compressive failure in multi-directional composite laminates is modelled by the tunnelling of a microbuckle within the load-bearing axial plies, with concomitant delamination of the neighbouring off-axis plies. The microbuckle tunnels at its tip in a crack-like mode III manner, and the steady state tunnelling stress is estimated by calculating the energy difference between the upstream unbuckled stale and the downstream buckled state. The downstream state is analysed in detail using a plane strain analysis of a microbuckle with delaminations from its tips. In the downstream 2D problem, microbuckling of the axial plies is represented by the generation of an inclined mode II crack, with an associated microbuckling tip toughness and a constant sliding stress across its flanks. The delaminations at the interface between the axial and adjacent off-axis plies are idealised as traction-free mixed-mode interfacial cracks. Predictions of the steady-state tunnelling stress are obtained for an isotropic solid by solving an integral equation and by the finite element method; finite element techniques are then used to solve the tunnelling problem for an orthotropic solid and for a cross-ply laminate. For each case, the tunnelling stress and the delamination crack length are obtained as functions of the ratio of delamination to microbuckle toughness, and of the inclination of the microbuckle band. The tunnelling stress provides a useful lower bound for the compressive strength of a thick laminated structure. (C) 2000 Elsevier Science Ltd. All rights reserved.